The present invention relates to an x-ray imaging system, an x-ray imaging method, and a computer-readable medium storing an x-ray imaging program for reconstructing an X-ray tomographic image of a subject at a given height thereof using projection data of X-ray images acquired by tomosynthesis imaging.
An X-ray imaging system for tomosynthesis imaging irradiates a subject with X ray at different angles while moving an X-ray source in one direction and detects the X ray with which the subject has been irradiated with a flat panel X-ray detector (FPD) to achieve acquisition of projection data corresponding to X-ray images of the subject taken at different angles by a single imaging operation. Then the process proceeds to image processing using the projection data corresponding to the X-ray images to reconstruct an X-ray tomographic image at a cross section of the subject at a given height thereof.
Now, reconstruction of an X-ray tomographic image will be described.
In tomosynthesis imaging, the X-ray source is moved in one direction to irradiate a subject 30 with X ray from positions S1, S2, and S3 as illustrated in FIG. 3A, so that two objects A, B are projected onto different positions in X-ray images (projection data) P1, P2, and P3 of the subject 30.
In the X-ray image P1, for example, the X-ray source, located in the position S1 to the left of the objects A, B in FIG. 3A, causes projections of the objects A, B to be formed in positions P1A, P1B that are set off to the right of the objects A, B. Likewise, in the X-ray image P2, the projections are formed in positions P2A, P2B that are substantially directly beneath the objects A, B; in the X-ray image P3, the projections are formed in positions P3A, P3B that are set off to the left of the objects A, B.
To reconstruct an X-ray tomographic image of the subject at a cross section located at a height of the object A, the X-ray image P1 is shifted leftward, and the X-ray image P3 is shifted rightward, for example, so that the projection positions P1A, P2A, and P3A coincide as illustrated in FIG. 3B (shift addition method). Thus, an X-ray tomographic image is reconstructed wherein the cross section located at the height of the object A is accentuated. An X-ray tomographic image at a cross section located at a given height, for example, at a height of the object B may likewise be reconstructed.
The FPD comprises photoelectric conversion elements arranged in matrix form and an X-ray receiving surface divided into sub-areas. The FPD has a data readout circuit in each sub-area. Therefore, to compensate for a variation in characteristics of photoelectric conversion elements, the data readout circuits or the like, various corrections are performed on projection data that are read out from the FPD.
JP 2007-632 A, for example, relates to compensation of an offset signal produced by a flat panel detector of a radiographic imaging apparatus. This literature describes an offset compensation for the flat panel detector using an offset map.
JP 10-327317 A, on the other hand, relates to a radiographic imaging apparatus capable of correcting an error contained in the imaging output caused by the difference between conditions under which data used for correction are obtained and conditions under which imaging is actually performed to provide image information having an enhanced signal-to-noise ratio.
Described in this literature is an imaging apparatus provided with an imaging means including photoelectric conversion elements one-dimensionally or two-dimensionally arranged, the imaging apparatus further comprising means for storing an imaging output in an imaging mode, means for storing imaging conditions in the imaging mode, means for obtaining a correction output in a correction mode operated using the stored imaging conditions, and correction means for correcting the imaging output using the correction output.
However, tomosynthesis imaging acquires projection data corresponding to a plurality of X-ray images, each of which has a large size, resulting in projection data having a large data quantity. Accordingly, where every image data having a large data quantity such as imaging data of X-ray images as acquired in tomosynthesis imaging was corrected, reconstruction of an X-ray tomographic image required a significantly, long time.
Further, some corrections, the same as those used in plain X-ray imaging, do not produce great effects upon the image quality of tomographic images whereas a number of other corrections are only effective when used in tomosynthesis imaging. Thus, while changing corrections according to imaging conditions, image data and requirements of users is effective, the inventions described in JP 2007-632 A and JP 10-327317 A were not so configured as to permit switching between corrections or selection of one of the corrections according to the conditions as described above.